Disk drives typically have disk synchronous repeatable runout (RRO), which is the offset between the ideal track location for a particular track and an actual track location, which is defined by servo burst information for the track formed on the disk. To accurately position a read or write head in the presence of such runout typically requires special RRO compensation algorithms. These algorithms inject a suitable control signal into a head actuator, so that the read or write head follows the desired RRO path with good accuracy. Because RRO is primarily induced by cyclically repeating phenomenon, such as disk eccentricity, clamping distortions, harmonic vibrations within the drive, and the like, the largest components of RRO error are harmonic in nature. Consequently, RRO compensation algorithms are designed to address the largest components of RRO by compensating for RRO that corresponds to various harmonics of disk rotational frequency.
Typically, RRO compensation algorithms minimize or eliminate RRO at a desired harmonic by determining the amplitude and phase of an appropriate sinusoidal compensation signal that is then injected into the head actuator. Alternatively, the sine and cosine components of the sinusoidal compensating signals may be used instead of amplitude and phase. In either case, such RRO compensation algorithms are typically adaptive, in that they continuously adjust the gain/phase or sin/cos compensation coefficients while the disk drive actuator servo system positions the read head on a certain disk drive track. However, the adaptation is typically slow, taking as many as several hundred disk revolutions to converge, and therefore is unable to converge to correct coefficient values when the read head is moved radially across the disk surface. As a result, the servo system cannot accurately compensate for RRO variation as the read head is moved across the surface of the disk radially, such as during seeks.
During some operations, it is desirable to minimize the magnitude of RRO while the head actuator is moving radially, i.e., moving across the stroke rather than servoing over a particular track. One such example is when a disk drive writes reference spirals on a disk as part of a spiral-based self-servo writing process. For an error-free and robust self-servo writing process, the reference spirals used should be precisely written on the disk surface, so that the drive can write servo wedges onto a surface of the disk with the necessary precision for proper operation of the drive. However, because existing RRO compensation algorithms may not provide sufficient head positioning accuracy while moving the head actuator radially to write reference spirals, such reference spirals can include an unacceptable level of error, thereby affecting operation of the drive. Accordingly, there is a need in the art for a method of accurately compensating for RRO when actuating a read or write head radially across a disk surface.